The module ThemeChanger not xoops version 2 works with me 2.5.7.2
System configuration

System Overview
XOOPS Version XOOPS 2.5.7.2
PHP Version 5.5.19
mySQL Version 5.6.21
Server API apache2handler
OS WINNT
safe_mode Off
register_globals Off
magic_quotes_gpc Off
allow_url_fopen On
fsockopen On
post_max_size 50M
max_input_time 60
output_buffering 4096
max_execution_time 30
memory_limit 128M
file_uploads On
upload_max_filesize 50M

This module is very old and has not been updated in quite some time. It would need to be updated before it can be used in any "modern" installation.

I looked over the code and it took about 30 seconds for me to recognize that I would not recommend installing this on a production site (or even a test site connected to the internet). There are some security concerns that must be addressed before this module is used.

There is a XOOPS core system block which will allow users to select one of the available themes. Maybe this is sufficient for what you need?